Several new monoclonal antibodies have been raised to leukemia-associated antigens. Some of these appear to be detecting differentiation antigens and are important for the phenotyping of leukemia. Among these are: (1)\CT2 which reacts with all E rosette-positive cells; (2)\CB2 which reacts with surface membrane immunoglobulin-positive cells; (3)\CG1 which reacts with granulocytes and CML; and (4)\CBL1 which reacts with blast cells. With these and other monoclonal antibodies it is now possible to phenotype leukemia by using a 2-hour microcytotoxicity test. We have also produced a monoclonal antibody against common ALL that cross reacts with platelets. It detects a cell membrane antigen of 26,000 daltons. CT2 and complement have been used successfully in 14 leukemic and immunodeficiency patients to remove T lymphocytes from allogeneic bone marrow. The incidence of graft-versus-host disease has been significantly reduced. We have produced two new monoclonal antibodies to T ALL that do not react to normal cells, including bone marrow stem cells. One of these was raised to non-primate cells transformed by oncogenic virus.